Prostatic acid phosphatase (PAP) is also known as orthophosphoric monoester phosphohydrolase, acid optimum, E.C. 3.1.3.2., and is a glycoprotein of about 100,000 daltons molecular weight normally found in the prostate gland. It is a major constituent of semen. It is also present in serum at low levels, and is normally a mixture of enzymes contributed by various tissues in the human body.
Assays for PAP began to be important several decades ago when it was discovered that serum acid phosphatase activity becomes elevated when metastatic cancer of the prostate or prostatomegaly is present. Thus, a determination of the amount of PAP in body fluids can be useful for the diagnosis of such diseases and for the observation of their progress or remission. Various spectrophotometric assays have been developed using immunological and non-immunological techniques and various enzyme substrates and dyes. Typically, PAP is evaluated from the amount or rate of formation of a chromophore (such as a dye). The chromophore is a coupled product of a phenol, resulting from the enzymatic hydrolysis of phospho monoester with, for example, a diazonium dye in an acidic environment.
A representative early assay for PAP is described in U.S. Pat. No. 3,002,893 (Babson). Another described by Babson et al in Am.J.Clin.Pathol., 32, pages 88-91 (1959) suffers from a number of disadvantages such as the need for a blank to be run, lack of kinetic results and difficulty in formulating a dry assay.
Other assays using various phosphate substrates have been developed, but suffer from insensitivity, reagent instability or nonspecificity for the analyte. The most difficult problem is the instability of various reagents which, in a dry element, are stored for lengthy periods of time prior to their use. For example, sodium salts of the PAP substrate are unstable when stored for a period of time.
It would be highly desirable to have an assay for PAP which can be carried out in a dry analytical element which would provide a number of important advantages. To achieve this result, however, requires that a number of technical problems (noted above) be overcome.